Sterile neutrino dark matter catalyzed by a very light dark photon

Sterile neutrinos (vs) that mix with active neutrinos (va) are interesting dark mat-ter candidates with a rich cosmological and astrophysical phenomenology. In their simplest incarnation, their production is severely constrained by a combination of structure formation observations and X-ray searches. We show that if active neutrinos couple to an oscillating condensate of a very light Li,- LT gauge field, resonant va-vs oscillations can occur in the early universe, consistent with vs constituting all of the dark matter, while respecting X-ray constraints on vs + va-y decays. Interesting deviations from standard solar and atmospheric neutrino oscillations can persist to the present.